Ion doping system, ion doping method and semiconductor device

ABSTRACT

An ion doping system includes a chamber  11 , an exhausting section  13  for exhausting gases from the chamber, an ion source  12  provided for the chamber, and an accelerating section  23  for extracting the ions, generated in the ion source  12 , from the ion source  12  and accelerating the ions toward a target. The ion source  12  includes an inlet port  14  to introduce a gas including a dopant element, a filament  15  emitting thermo electrons, and an anode electrode  17  to produce an arc discharge between the filament and itself. The ion source  12  decomposes the gas through the arc discharge, thereby generating ions including the dopant element. The ion doping system controls the arc discharge such that a constant amount of arc current flows between the filament and the anode electrode.

TECHNICAL FIELD

The present invention relates to an ion doping system for implantingions into a target (i.e., doping the target with the ions). The presentinvention also relates to a semiconductor device fabricated by usingsuch an ion doping system.

BACKGROUND ART

Recently, to realize a big high-resolution liquid crystal display, ahigh-speed high-resolution close-contact image sensor, athree-dimensional IC and so on, people have tried to fabricate ahigh-performance semiconductor component on an insulating substrate ofglass, for example.

To make such a semiconductor component, a thin-film semiconductor layerneeds to be formed. Thus, techniques of fabricating a semiconductorcomponent on an insulating substrate using either a semiconductor thinfilm of an amorphous silicon (a-Si) semiconductor or a semiconductorthin film of a crystalline silicon semiconductor such as polysilicon ormicrocrystalline silicon are known in the art.

The amorphous silicon semiconductor thin film can be formed at arelatively low temperature by a vapor phase process, and therefore, ismass-producible and generally used most commonly. However, the amorphoussilicon semiconductor thin film has inferior physical properties interms of conductivity, for example. That is why if a device of any ofthose various types requires high-performance semiconductor components,the amorphous silicon semiconductor thin film cannot be used effectivelyfor that purpose.

Meanwhile, the crystalline silicon semiconductor thin film has superiorconductivity and has been the object of numerous researches anddevelopments to use the film in those devices. Following methods ofmaking a thin-film crystalline silicon semiconductor are known:

-   -   (1) directly form a silicon semiconductor film with        crystallinity during its film deposition process;    -   (2) form an amorphous silicon semiconductor film once and then        expose it to intense light, thereby crystallizing the amorphous        silicon with its energy; and    -   (3) form an amorphous silicon semiconductor film once and then        apply thermal energy to it, thereby crystallizing the amorphous        silicon.

According to the method (1), silicon is crystallized as the filmdeposition process proceeds. That is why unless the film being depositedis sufficiently thick, crystalline silicon with a large crystal grainsize will not be obtained. Consequently, according to this method, it istechnically difficult to form a crystalline silicon semiconductor thinfilm with good semiconductor properties over the entire surface of asubstrate with a broad area. On top of that, since the film needs to bedeposited at a temperature of 600° C. or more, an inexpensive glasssubstrate with a low softening point cannot be used as an insulatingsubstrate to raise the manufacturing cost unintentionally.

(2) According to the method (2), a crystallization phenomenon during amelting and solidifying process is utilized, and therefore, high-qualitycrystalline silicon semiconductor with a small crystal grain size but awell processed grain boundary can be obtained. However, a practicalmeans for irradiating a substrate with a broad area with such intenselight is rare to find. For example, when an excimer laser, which iscurrently used most commonly, is used, the laser beam does not have sogood stability as to process the entire surface of a substrate with abroad area uniformly. Thus, it is hard to obtain a crystalline siliconfilm according to this method, too. Consequently, it is difficult tofabricate a plurality of semiconductor components with uniformcharacteristics over the same substrate. What is worse, since the laserbeam cannot cover the broad area, the productivity is bad.

Compared to the methods (1) and (2), the method (3) is advantageous inthat a crystalline silicon semiconductor film can be deposited over abroad area relatively easily. However, to complete the crystallization,a heat treatment must be conducted at as high a temperature as 600° C.or more for several tens of hours. Accordingly, if the heatingtemperature is lowered to use an inexpensive glass substrate, the heattreatment needs to be carried out for even a longer time, thusdecreasing the throughput. Also, since a solid phase crystallizationphenomenon is utilized according to this method, the crystal grains willgrow parallel to the substrate plane and their crystal grain sizes maysometimes reach several μm. However, since a grain boundary is formed asa result of contact between grown crystal grains, the grain boundaryserves as a trap level against carriers, which may cause a decrease inelectron mobility.

Among these three methods, people are paying particularly much attentionto the method (3) as one of most promising methods. Thus, a method offorming a high-quality, highly uniform crystalline silicon film bycarrying out a heat treatment at a low temperature and in a short timeby utilizing the method (3) is disclosed in Japanese Patent ApplicationLaid-Open Publications Nos. 6-333824, 6-333825 and 8-330602, forexample.

According to the method disclosed in these patent documents, it would bepossible to complete the crystallization at as low a temperature as 600°C. or less and in a processing time of about a few hours by thermallytreating an amorphous silicon film with a very small amount of nickel orany other metallic element introduced through the surface of theamorphous silicon film.

In this method, first, nuclei of crystals are generated from theintroduced metallic element at an early stage of the heat treatmentprocess, and then the metallic element functions as a catalyst forpromoting the crystal growth of silicon, thus advancing thecrystallization rapidly. That is why the metallic element introduced iscalled a “catalyst element”. A silicon film that has been crystallizedby a normal solid phase growth process has a twin crystal structure,whereas a crystalline silicon film, obtained by this method, consists ofa great many columnar crystals. And the inside of each of those columnarcrystals almost has a single crystal state.

According to this method, however, if the catalyst element remained inthe silicon film, then normal semiconductor component characteristicswould not be achieved. For that reason, as disclosed in Japanese PatentApplication Laid-Open Publication No. 6-333824 or No. 8-236471, thecatalyst element is trapped using phosphorus ions, for example. Morespecifically, after a crystalline silicon film, obtained by a heattreatment using a catalyst element introduced, has been patterned, agate insulating film is deposited on the surface of the crystallinesilicon film and then a gate electrode is formed thereon. Then, thepatterned crystalline silicon film is doped with phosphorus ions usingthe gate electrode as a mask. As a result, portions of the crystallinesilicon film (i.e., source/drain regions), except the region right underthe gate electrode, are doped with phosphorus. Thereafter, thephosphorus ions introduced are activated with either thermal energy orlaser light. As a result, the catalyst element that has been located inthe region right under the gate electrode are trapped (i.e., gettered)in the source/drain regions, and a thin-film transistor, using theregion right under the gate electrode as a channel region, can beobtained.

The phosphorus ion doping process needs to be carried out on acrystalline silicon film with a broad area. For that purpose, an ionbeam system that can emit an ion beam with a large cross section isused. To generate a lot of ions and to form an ion beam with a broadcross section, such an ion beam system uses diborane and phosphine assource gases, generates an ion beam by getting these gases decomposed byan ion source, and then irradiates a crystalline silicon film with thation beam without passing the beam through a mass separator. In thiscase, the conventional ion beam system is controlled such that the ionbeam has a constant beam current density and the crystalline siliconfilm is doped with the ions.

When such a control is carried out, however, the quantity of charge perunit area of the doping ions becomes constant but the species of theions generated is variable. The properties of a semiconductor dopedchange according to the ion species. That is why if semiconductorcomponents are fabricated by getting a crystalline silicon film dopedwith ions by a conventional ion beam system, then the characteristics ofthe semiconductor components will vary significantly. In addition, dueto such a significant variation in characteristics, the yield ofsemiconductor devices that satisfy a predetermined standard decreases,too.

DISCLOSURE OF INVENTION

In order to overcome the problems described above, an object of thepresent invention is to provide an ion doping system and ion dopingmethod, in which the ratio of ion species generated varies to a muchlesser degree. The present invention also relates to a semiconductordevice, fabricated by using such an ion doping system, and a method forfabricating such a semiconductor device.

An ion doping system according to the present invention includes: achamber; an exhausting section for exhausting gases from the chamber; anion source, which is provided for the chamber, includes an inlet port tointroduce a gas including a dopant element, a filament emitting thermoelectrons, and an anode electrode to produce an arc discharge betweenthe filament and itself, and decomposes the gas through the arcdischarge, thereby generating ions including the dopant element; and anaccelerating section for extracting the ions, generated in the ionsource, from the ion source and accelerating the ions toward a target.The ion doping system controls the arc discharge such that a constantamount of arc current flows between the filament and the anodeelectrode.

In one preferred embodiment, the ion doping system further includes: afilament power supply for applying a voltage to the filament; and an arcpower supply for applying a voltage between the filament and the anodeelectrode. The ion doping system controls the filament power supplyand/or the arc power supply such that a constant amount of arc currentflows between the filament and the anode electrode.

In another preferred embodiment, the ion doping system further includesan ammeter for measuring the arc current and controls the output voltageof the arc power supply such that the arc current measured with theammeter has a constant value.

In still another preferred embodiment, a plurality of ions species areproduced at a constant ratio from the gas as a result of the arcdischarge.

A semiconductor device according to the present invention includes: asubstrate with an insulating surface; and a crystalline silicon filmprovided on the substrate. The semiconductor device includes a pluralityof semiconductor components, in each of which source, drain and channelregions are defined in the crystalline silicon film by introducing thedopant element as an impurity into the crystalline silicon film using anion doping system according to any of the preferred embodimentsdescribed above.

In one preferred embodiment, the semiconductor device satisfies theinequality 0.05≧3 σ/Ave, where Ave is the average of dopantconcentrations in the respective channel regions of the semiconductorcomponents and σ is the standard deviation thereof.

In an alternative preferred embodiment, the semiconductor devicesatisfies the inequality 0.05≧3 σ/Ave, where Ave is the average ofdopant concentrations in the respective source/drain regions of thesemiconductor components and σ is the standard deviation thereof.

In another preferred embodiment, the crystalline silicon film has beencrystallized with a catalyst element that promotes the degree ofcrystallinity of an amorphous silicon film.

In this particular preferred embodiment, the amorphous silicon film hasa thickness of 25 nm to 80 nm.

In another preferred embodiment, the crystalline silicon film has thecatalyst element at a concentration of 1×10¹⁶ atoms/cm³ or less.

In still another preferred embodiment, the catalyst element is at leastone element selected from the group consisting of nickel, cobalt,palladium, platinum, copper, silver, gold, indium, tin, aluminum andantimony.

In a specific preferred embodiment, the catalyst element is nickel.

In yet another preferred embodiment, the crystalline silicon film hasbeen formed by carrying out at least one of a furnace heating process, alamp annealing process and a laser radiation process after the catalystelement has been introduced.

An ion doping method according to the present invention includes thesteps of: decomposing a gas, including a dopant element, through an arcdischarge; and accelerating ions, which have been produced as a resultof the step of decomposing, with a predetermined voltage, therebybombarding a target with the ions. The step of decomposing includescontrolling the arc discharge such that a constant amount of current isproduced by the arc discharge.

In one preferred embodiment, the dopant element is boron or phosphorus.

A method for fabricating a semiconductor device according to the presentinvention includes the steps of: (A) forming an amorphous silicon filmon a substrate with an insulating surface; (B) adding a catalyst elementto the amorphous silicon film; (C) thermally treating and crystallizingthe amorphous silicon film, to which the catalyst element has beenadded, thereby turning the amorphous silicon film into a crystallinesilicon film; (D) generating an arc discharge that produces a constantamount of arc current, thereby decomposing a gas including an impurityelement and accelerating, and introducing into the crystalline siliconfilm, ions that have been generated as a result of the decomposition;and (E) thermally treating the crystalline silicon film.

In one preferred embodiment, the method further includes the step offorming an insulating film on the crystalline silicon film after thestep (C) has been performed. The step (D) includes the steps of: (D1)generating the arc discharge that produces the constant amount of arccurrent, thereby decomposing the gas including the impurity element andaccelerating, and introducing into the crystalline silicon film by wayof the insulating film, the ions that have been generated as a result ofthe decomposition; (D2) making a pattern of a conductive material on thecrystalline silicon film; and (D2) generating the arc discharge thatproduces the constant amount of arc current, thereby decomposing the gasincluding the impurity element and accelerating, and introducing intothe crystalline silicon film using the pattern as a mask, the ions thathave been generated as a result of the decomposition.

In one preferred embodiment, the impurity element in the step (D1) isboron and the impurity element in the step (D2) is phosphorus.

In another preferred embodiment, the catalyst element is at least oneelement selected from the group consisting of nickel, cobalt, palladium,platinum, copper, silver, gold, indium, tin, aluminum and antimony.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation illustrating a preferred embodimentof an ion doping system according to the present invention.

FIG. 2 is a graph showing how the ratio of ion species changes with thearc current.

FIGS. 3(a) through 3(e) are schematic cross-sectional views illustratingrespective process steps for fabricating a semiconductor deviceaccording to the present invention.

FIG. 4 is a schematic representation illustrating another preferredembodiment of an ion doping system according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 illustrates a preferred embodiment of an ion doping systemaccording to the present invention. The ion doping system 10 shown inFIG. 1 includes a chamber 11, an exhausting section 13, an ion source 12and an accelerating section 23.

The exhausting section 13 may be implemented as a known pump, forexample, and can exhaust gases from the chamber 11 and maintain anappropriate pressure within the chamber. Although not shown, adisinfecting system for removing poisons from various gases used in theion doping system is connected to the exhausting section 13.

The ion source 12 decomposes a gas including a dopant element through anarc discharge, thereby generating plasma and ions including the dopantelement. The ion source 12 is arranged within the chamber 11 andincludes an arc chamber 30, gas inlet ports 14, and filaments 15. Thegas inlet ports 14 are provided for the arc chamber 30 to introduce agas including the dopant element into the arc chamber 30 of the ionsource 12. The filaments 15 are arranged on the ceiling of the arcchamber 30. In FIG. 1, three filaments 15 are shown. However, any otherappropriate number of filaments may be arranged elsewhere according tothe shape of the arc chamber 30 and the shape and size of an ion beamrequired. An anode electrode 17 is provided on the side surface of thearc chamber 30. Around the arc chamber 30, arranged are ring magnets 31for confining the generated plasma in the arc chamber 30.

A filament power supply 16 is connected to each of the filaments 15. Theion doping system 10 includes a controller 27 such as a microcomputerfor controlling the output voltages of the filament power supplies 16and the amount of currents flowing through the filaments 15.

Also, an arc power supply 20 is connected between the filaments 15 andthe arc electrode 15. An ammeter 19 is preferably provided between thefilaments 15 and the arc power supply 20. The value measured by theammeter 19 is input to the controller 27. The arc power supply 20 isalso controlled by the controller 27.

The accelerating section 23 extracts the ions, generated by the ionsource 12, from the ion source 12 and accelerates those ions toward atarget. The ions generated by the ion source 12 are all accelerated bythe accelerating section 23 without being separated by any massseparator. For that purpose, the accelerating section 23 includesextracting electrodes 18, an extracting power supply 21 and anaccelerating power supply 22. The extracting electrodes 18 include afirst extracting electrode 18 a and a second extracting electrode 18 band are arranged at the opening of the arc chamber 30. The firstextracting electrode 18 a is located closest to the opening of the arcchamber 30 and the extracting power supply 21 is connected between thefirst extracting electrode 18 a and the arc electrode 17. Also, theaccelerating power supply 22 is connected between the first and secondextracting electrodes 18 a and 18 b.

A substrate 25, i.e., the target into which the ions should beintroduced, is held by a substrate holder 28 so as to face the openingof the arc chamber 30 with the extracting electrodes 18 interposedbetween them. If the ion beam 33 emitted from the ion source 12 cannotirradiate the substrate 25 at a time due to its shape, then a mechanismfor shifting the substrate 25 such that the ion beam 33 can scan thesubstrate 25 may be provided. A beam current meter 26 is provided underthe substrate 25 so as to measure the current produced by the ion beam33.

Hereinafter, the operation of the ion doping system 10 and an ion dopingprocess using the ion doping system 10 will be described. First, thesubstrate 25 is fixed with the substrate holder 28 at such a position inthe chamber 11 as to be irradiated with the ion beam 33. Thereafter, thechamber 11 is evacuated until a predetermined degree of vacuum iscreated. A semiconductor film, to which dopant ions should beintroduced, may have been deposited on the surface of the substrate 25,for example.

Next, a gas including an element to be dopant ions is introduced throughthe gas inlet ports 14. For example, in introducing phosphorus as n-typedopant ions into a silicon semiconductor as a target, phosphine (PH₃) isused. On the other hand, in introducing boron as p-type dopant ions intothe target, diborane (B₂H₆) is used. As a result, the arc chamber 30 isfilled with a low-pressure gas including an element to be dopant ions.

The filament power supplies 16 apply a predetermined voltage to thefilaments 15, thereby supplying the filaments 15 with current. And thearc power supply 20 applies another predetermined voltage between thefilaments 15 and the arc electrode 17. Then, thermo electrons, heated bythe filaments 15, are emitted from the filaments 15 into the arc chamber30 to reach the arc electrode 17. These electrons emitted produce an arcdischarge. The emitted thermo electrons collide against the phosphine ordiborane molecules in the arc chamber 30, thereby decomposing andionizing these molecules. As a result, plasma is generated.

The controller 27 of the ion doping system 10 controls the arc dischargesuch that a constant amount of arc current flows between the filaments15 and the arc electrode 17. More particularly, the controller 27controls the filament power supply 20 and the arc power supply 21according to the value of current measured by the ammeter 19 to make theamount of arc current flowing constant. As a result, the amount ofcurrent flowing through the filaments 15 and/or the arc voltage betweenthe filaments 15 and the arc electrode 17 are regulated and the quantityof thermo electrons emitted from the filaments per unit time can be keptconstant. That is to say, the same quantity of thermo electrons aresupplied with respect to the gas that is introduced at a constant ratethrough the gas inlet ports 14. As a result, the gas decomposition canmaintain a constant state and the ratio of ion species generated becomesconstant, too.

The cations in the generated plasma are extracted through the opening ofthe arc chamber 30 responsive to the voltage that has been applied bythe extracting power supply 21 between the arc electrode 17 and thefirst extracting electrode 18 a. The extracted cations are accelerateddue to the voltage applied by the accelerating power supply 22 betweenthe first and second extracting electrodes 18 a and 18 b. Then, theaccelerated cations impinge as the ion beam 33 onto the substrate 25.The charge carried by the ion beam 33 is measured by the beam currentmeter 26, thereby obtaining the current or the current density of theion beam 33.

FIG. 2 shows how the ratio of respective ion species generated changeswith the amount of arc current in a situation where diborane isdecomposed by using the ion doping system 10. The abscissa representsthe amount of arc current per filament, while the ordinate representsthe ratio of generated ion species in percentages. In FIG. 2, “B1 based”refers to a chemical species including a single boron atom (such as BH⁺and BH₂ ⁺), “B2 based” refers to a chemical species including two boronatoms (such as B₂H⁺ and B₂H₂ ⁺), and “H based” refers to a chemicalspecies including only hydrogen atom(s) (such as H⁺ and H₂ ⁺). As isclear from FIG. 2, as the amount of arc current increases, the “B1based” and “H based” chemical species increase and the “B2 based”chemical species decreases. In other words, as the amount of arc currentchanges, the ratio of these ion species generated varies, too.

Accordingly, if the ion doping process is controlled so as to make theion beam current density constant as in the conventional ion dopingsystem, then the amount of arc current changes and the ratio of ionspecies generated also varies. This means that even if the ion dopingprocess is carried out while being controlled to make the ion beamcurrent density constant, the ratio of generated ion species stillvaries and the implant dose of the dopant ions changes, too. Forexample, even if the ion beam current density remains the same but ifthe amount of arc current per filament has changed from 1 mA into 2 mA,then the implant dose of boron decreases by a factor of 0.84(=(14×2+14)/(20×2+10)). Also, if the ratio of ion species generatedchanges during the ion doping process, the distribution of the ionspecies in the arc chamber changes, too, thus making the distribution ofthe ion species in the ion beam non-uniform. Consequently, if the targetis doped with ions with the ion beam current density set constant, thedopant ions will have a non-uniform in-plane distribution.

In contrast, if the target is doped with ions with the amount of arccurrent set constant by using the ion doping system of the presentinvention, the ratio of ion species generated can be kept constant. As aresult, the distribution of the ion species in the arc chamber can alsobe kept constant and the ion species are distributed uniformly in theion beam. Consequently, if the target is doped with ions with the amountof arc current set constant, the dopant ions will have a uniformin-plane distribution.

It should be noted that if the arc discharge is controlled so as to makethe amount of arc current constant, then the beam current density of theresultant ion beam changes. However, the present inventors discoveredand confirmed via experiments that the variation in beam current densitywas small enough to match the overall doses by adjusting theimplantation process times. As a result, although the implantationprocess time may change to a certain degree, the ratio of the ionspecies implanted into the target can be kept constant and the ionsspecies introduced into the target will have a higher degree of in-planeuniformity.

Consequently, if dopant ions are implanted into a crystalline siliconfilm for a liquid crystal display with the amount of arc current setconstant by using the ion doping system of the present invention, thenvariations in threshold voltage and in semiconductor property such as asource-drain resistance can be minimized among a plurality of thin-filmtransistors fabricated on a substrate plane.

Hereinafter, it will be described how to fabricate a semiconductordevice using the ion doping system of the present invention. In thefollowing description, it will be described how to make an active-matrixsubstrate for an LCD by fabricating a huge number of n-channel TFTs(thin-film transistors) as pixel switching elements on a glasssubstrate. Several hundreds of thousands of to a few million TFTs areusually fabricated on the same substrate and should have uniformcharacteristics. The ion doping system of the present invention can beused effectively to make such an active-matrix substrate for an LCD. Ifnecessary, not just those pixel switching elements but also the elementsforming the driver or thin-film integrated circuit of the active-matrixsubstrate may also be fabricated efficiently by using the ion dopingsystem of the present invention.

FIGS. 3(a) through 3(e) show respective manufacturing process steps offabricating a semiconductor device according to the present invention,including a great many n-channel TFTs, in the order in which thoseprocess steps are carried out. Although at least several hundreds ofthousands of TFTs are actually fabricated, it will be described how tofabricate just one of them.

First, as shown in FIG. 3(a), an undercoat film 102 of silicon dioxideis deposited by a plasma CVD process to a thickness of 1 nm to 20 nm onan insulating substrate 101 made of glass, for example. Next, anintrinsic amorphous silicon film 103 is deposited thereon to a thicknessof 25 nm to 80 nm (e.g., 40 nm) by a plasma CVD process.

Subsequently, Ni is added as a catalyst element to the intrinsicamorphous silicon film 103 by a sputtering process so as to have asurface density of 1×10¹³ atoms/cm² to 1×10¹⁵ atoms/cm² (e.g., 7×10¹³atoms/cm²). Thereafter, the substrate 101 is subjected to a heattreatment at a temperature of 540° C. to 620° C. for several hourswithin an inert atmosphere by using a heat treatment furnace or a lampannealing furnace, for example. As a result of this heat treatment, thecrystallization of the intrinsic amorphous silicon film 103 proceeds. Inthis preferred embodiment, the heat treatment is carried out at 580° C.for an hour within a nitrogen atmosphere by using a heat treatmentfurnace. Ni does not have to be added by the sputtering process.Alternatively, the intrinsic amorphous silicon film 103 may be coatedwith a coating solution including a Ni compound, which may then bethermally treated to diffuse Ni from the coating into the intrinsicamorphous silicon film 103 and promote the crystallization of theintrinsic amorphous silicon. Examples of preferred catalyst elementsother than Ni include cobalt, palladium, platinum, copper, silver, gold,indium, tin, aluminum and antimony. Optionally, a plurality of metalsselected from this group may be used in combination.

Next, as shown in FIG. 3(b), the amorphous silicon film is crystallizedby being exposed to a laser beam. As the laser beam, a KrF excimer laserbeam with a wavelength of 248 nm and a pulse width of 20 nsec may beused. Alternatively, a laser beam with any other wavelength may also beused. In any case, the laser beam is radiated at an energy density of200 mJ/cm² to 400 mJ/cm² (e.g., at 250 mJ/cm²) and one location is shotwith the laser beam twice to ten times (e.g., twice). Optionally, inradiating the laser beam, the substrate may be heated to a temperatureof about 200° C. to about 450° C. By heating the substrate during thelaser beam radiation process in this manner, the crystallization of theamorphous silicon can be promoted even more efficiently.

During these two-stage heat treatment processes, the catalyst elementturns into a silicide, thereby promoting the crystallization of theamorphous silicon film. Among other things, the crystal structure ofNiSi₂, which is a silicide compound of nickel, is closest to that ofsingle crystalline silicon than the silicide compound of any othercatalyst element. And its lattice constant is also very close to that ofcrystalline silicon. Accordingly, NiSi₂ functions as the best mold tocrystallize the amorphous silicon film and does promote thecrystallization of the amorphous silicon film. In this process step, bysetting the thickness of the amorphous silicon film 103 equal to orgreater than 25 nm, sufficient crystal growth is realized. And bysetting the thickness equal to or smaller than 80 nm, it is possible toprevent crystals from growing in two or more layers in the thicknessdirection. As a result, the degraded crystallinity, residual catalystelement and other problems can be avoided, and a high-qualitycrystalline silicon film 103′ with a high electron mobility can be madefrom the amorphous silicon film 103. The heat treatment for the purposeof crystallization may be carried out in a single stage. However, it ismore preferable to combine the heat treatment using a heat treatmentfurnace or a lamp annealing furnace with the heat treatment using alaser radiation. By performing the heat treatments in these two stages,the transistor characteristics of the resultant TFTs will improvesignificantly.

Thereafter, as shown in FIG. 3(c), excessive portions of the crystallinesilicon film 103′ are removed to electrically isolate the respectiveelements from each other and define island-like element forming areas,each including the source, drain and channel regions of a thin-filmtransistor. On the overall substrate 101, a huge number of elementforming areas 115 are arranged in matrix.

Next, as shown in FIG. 3(c), a silicon dioxide film 104 is deposited asa gate insulating film to a thickness of 50 nm to 250 nm (e.g., 150 nm)by a plasma CVD process and then boron ions are implanted into theelement forming area 115 through the silicon dioxide film 104 by usingthe ion doping system 101 shown in FIG. 1. In this preferred embodiment,the ion doping system 101 includes four filaments 15, and the dopingprocess is carried out with the amount of arc current flowing througheach filament maintained at a constant value between 1.5 mA and 2.5 mA.For example, the ion doping system 101 may be controlled such that anarc current of 2.0 mA flows per filament and boron ions may be implantedat a dose of 5×10¹¹ cm⁻² to 5×10¹³ cm⁻² into the element forming area115. Diborane may be used as the source of boron.

Subsequently, as shown in FIG. 3(d), tantalum nitride (TaN) and tungsten(W) are deposited to a thickness of 10 nm to 100 nm (e.g., 60 nm) and toa thickness of 100 nm to 500 nm (e.g., 300 nm), respectively, by asputtering process. And by patterning these films deposited, a gateelectrode of TaN/W is formed.

Thereafter, phosphorus ions are implanted into the element forming area115 by using the ion doping system 101 and using the gate electrode 105as a mask. Phosphine (PH₃) may be used as the doping gas, theaccelerating voltage may be 60 kV to 90 kV (e.g., 80 kV) and the implantdose may be 1×10¹⁵ cm⁻² to 8×10¹⁵ cm⁻² (e.g., 2×10¹⁵ cm⁻²). The iondoping system 101 is controlled such that the amount of arc currentflowing becomes a constant value falling within the range of 400 mA to500 mA (e.g., 450 mA) while the phosphorus ions are being implanted.

After the phosphorus ions have been implanted, the substrate isthermally treated at 550° C. for four hours within a nitrogenatmosphere, thereby activating the dopant introduced. In the meantime,nickel atoms in a portion of the element forming area 115 under the gateelectrode 105 (i.e., a region to be a channel region 107) are trapped bythe phosphorus atoms in portions of the element forming area 115 thathave been doped with phosphorus ions (i.e., regions to be source/drainregions 106, 108). As a result, the concentration of nickel in thatregion under the gate electrode 105 decreases to about 1×10¹⁶ atoms/cm³.

In this manner, a p-type channel region 107 is defined in that portionof the element forming area 115 under the gate electrode 105 and n-typesource/drain regions 106 and 108 are defined in the element forming area115 so as to interpose the channel region 107 between them as shown inFIG. 3(d). That is to say, an n-channel TFT 116, including the gateelectrode 105, channel region 107, and source/drain regions 106 and 108,is completed. Since the concentration of nickel in the channel region107 is 1×10¹⁶ atoms/cm³ or less as described above, the TFT 116 has asmall amount of leakage current and high crystallinity in its channelregion 107 and therefore has a large amount of ON-state current.

In making a complementary circuit including n-channel and p-channelTFTs, the crystalline silicon film 103′ may be selectively doped withboron and phosphorus through appropriate masks such that n-type andp-type regions are defined separately and that an n-channel TFT and ap-channel TFT are formed on the same substrate.

Next, as shown in FIG. 3(e), a silicon dioxide film 109 is deposited toa thickness of 600 nm as an interlayer dielectric film by a plasma CVDprocess, and contact holes are opened through the silicon dioxide film.One of them is filled with a multilayer film of metals (e.g., titaniumnitride and aluminum) to form an electrode 110 for the thin-filmtransistor and another hole is filled with ITO to make a pixel electrode111. Finally, a heat treatment is conducted at 350° C. for 30 minuteswithin a hydrogen atmosphere at 1 atmospheric pressure, therebycompleting a semiconductor device including a plurality of TFTs 116.

The following Table 1 shows variations in TFT characteristics on anactive-matrix substrate that was made by the method described above andon an active-matrix substrate that was made by the manufacturing processdescribed above except that boron and/or phosphorus were implanted by aconventional control technique that sets the amount of ion beam currentconstant. As the variations in TFT characteristics, variations inthreshold voltage were calculated and are shown in percentages, and theaverages (Ave) and standard deviations (σ) of the source-drainresistance within the substrate plane were calculated and 3 σ/Ave areshown in percentages. The percentages of resultant active-matrixsubstrates, of which the variations in TFT characteristics satisfiedpredetermined criteria, are also shown. The active-matrix substrates haddimensions of 60 mm×80 mm and included one million TFTs. TABLE 1Characteristics (variation) Control method Source- Boron Phosphorusdrain Sample doping doping Threshold resistance No. (channel)(source/drain) voltage 3 σ/Ave Yield 1 Arc Arc current 3.8% 3.4% 98%current control control 2 Arc Beam current 3.9% 6.8% 94% current controlcontrol 3 Beam Arc current 7.3% 3.5% 92% current control control 4 BeamBeam current 7.4% 6.7% 80% current control control

As is clear from the results of Sample No. 1 shown in Table 1, if theion doping system was controlled such that the amount of arc currentflowing became constant both when boron was implanted and whenphosphorus was implanted, the variation in threshold voltage and thevariation (3 σ/Ave) in source-drain resistance within the substrateplane were as small as less than 5% and the yield of substrates was ashigh as 98%. The present inventors discovered via intensive experimentsthat by using the ion doping system of the present invention, thevariation in dopant concentration (i.e., 3 a/Ave when the average (Ave)and standard deviation (σ) of the dopant concentration were calculated)could be reduced to less than 5% with respect to an area of 1,000mm×1,000 mm.

On the other hand, if the ion doping system was controlled as in theconventional process (i.e., such that the amount of beam current flowingbecame constant) when boron or phosphorus was implanted, the variationin threshold voltage or the variation in source-drain resistanceincreased to about 7% and the yield decreased. Among other things, ifthese implantation processes were both carried out by controlling thesystem by the conventional method to make the amount of beam currentconstant, the variation in threshold voltage and the variation insource-drain resistance both increased. As a result, the yield droppedto 80%.

Thus, according to this preferred embodiment, when a dopant isintroduced into a semiconductor film using an ion doping system, the iondoping system is controlled such that the arc current of arc dischargeto generate plasma at an ion source flows in a constant amount. As aresult, the ratio of ion species generated at the ion source becomesconstant and the ion doping process can be controlled precisely.

Particularly when a TFT is fabricated by using a crystalline siliconfilm, which is obtained by crystallizing an amorphous silicon film witha catalyst element added, the doping level of phosphorus ions introducedinto the source/drain regions has a significant effect on thecharacteristic of the resultant TFT. More specifically, if the dopinglevel of the phosphorus ions is lower than its setting, then the carrierdensity decreases and the source-drain resistance increases. Inaddition, since the concentration of phosphorus will not be high enoughto trap the catalyst element that promotes the crystallization ofamorphous silicon, the catalyst element will remain a lot in the channelregion, the amount of leakage current will increase and other TFTcharacteristics will deteriorate. As a result, the TFT may cause afailure during its operation. On the other hand, if the doping level ofphosphorus ions is higher than its setting, then the phosphorus ionswill be implanted excessively to destroy the crystal structure ofcrystallized silicon and amorphize the crystallized silicon.Consequently, the source-drain resistance rises.

The ion doping system of the present invention can keep the ratio of theion species generated quite constant during the ion doping process asdescribed above. Thus, the ion species implanted can have a uniformdistribution over the entire substrate and the ratio of the ion speciesgenerated can be kept constant during the ion doping process. That iswhy by getting phosphorus ions doped by using the ion doping system ofthe present invention and getting the implant dose measured by a beamcurrent meter, the implant dose of the phosphorus ions can be controlledprecisely and a TFT can be fabricated by using a crystalline siliconfilm obtained by crystallizing an amorphous silicon film with a catalystelement.

In addition, since the ratio of the ion species can be kept constantduring the ion doping process, the ion species are also distributeduniformly in the ion beam and the ions implanted will have an increaseddegree of in-plane uniformity. As a result, in the semiconductor deviceof the present invention, the ratio and the doses of the dopant ionsintroduced into the semiconductor film can be made uniform and thevariations in characteristics among a plurality of semiconductorcomponents can be reduced.

In the preferred embodiment described above, in fabricating a TFT usinga crystalline silicon film that has been obtained by crystallizing anamorphous silicon film with a catalyst element, boron and phosphorus areimplanted to define a channel and source/drain regions, respectively, byusing the ion doping system of the present invention. However, the iondoping system of the present invention may also be used in a dopingprocess step to fabricate a semiconductor device of a different type.For example, if TFTs on an active-matrix substrate have an LDDstructure, the ion doping system of the present invention may be used ina doping process step to define the LDD structure. As anotheralternative, the ion doping system of the present invention may also beused to fabricate various types of semiconductor devices on a singlecrystalline semiconductor substrate.

Also, in the ion doping system shown in FIG. 1, the accelerating section23 is made up of two electrodes. However, the number of electrodes toprovide does not have to be two. For example, in an alternative iondoping system 10′ shown in FIG. 4, the accelerating section 23 includesan electrode 18, an extracting power supply 21, an accelerating powersupply 22, and a decelerating power supply 32. The electrode 18 includesfirst, second, third and fourth electrodes 18 a, 18 b, 18 c and 18 d,which are all provided around the opening of the arc chamber 30. Thefirst electrode 18 a is located closest to the opening of the arcchamber 30 and the extracting power supply 21 is connected between thefirst electrode 18 a and the arc electrode 17.

A power supply 22 a is connected between the first and second electrodes18 a and 18 b and another power supply 22 b is connected between thesecond and third electrodes 18 b and 18 c. And the decelerating powersupply 32 is connected between the third and fourth electrodes 18 c and18 d. The power supply 22 a applies a voltage to extract ions, generatedin the ion source 12, between the first and second electrodes 18 a and18. On the other hand, the power supply 22 b applies a voltage toaccelerate the extracted ions between the second and third electrodes 18b and 18 c. In this case, the sum of the voltages applied by the powersupplies 22 a and 22 b is generally called an “accelerating voltage”.Meanwhile, the decelerating power supply 32 prevents secondary electronions, generated by the ions that have collided against, or beenimplanted into, the substrate 25, from being accelerated by theaccelerating section 23 toward the ion source 12.

Optionally, as shown in FIG. 4, a shifting mechanism 29 for shifting thesubstrate holder 28 may also be provided for the ion doping system andmay shift the substrate 25 such that the ion beam 33 scans the substrate25.

INDUSTRIAL APPLICABILITY

According to the present invention, an ion doping system, which canminimize the variation in the ratio of ion species generated andrealizes a higher degree of controllability, can be provided. This iondoping system can be used particularly effectively to make asemiconductor device with a large area as a display device, for example.In addition, according to the present invention, a semiconductor devicethat hardly exhibits variations in device characteristics within thesubstrate plane can be provided. Such a semiconductor device can be usedin various applications and is particularly effectively applicable tomake a semiconductor device with a large area as a display device, forexample.

1. An ion doping system comprising: a chamber; an exhausting section forexhausting gases from the chamber; an ion source, which is provided forthe chamber, includes an inlet port to introduce a gas including adopant element, a filament emitting thermo electrons, and an anodeelectrode to produce an arc discharge between the filament and itself,and decomposes the gas through the arc discharge, thereby generatingions including the dopant element; and an accelerating section forextracting the ions, generated in the ion source, from the ion sourceand accelerating the ions toward a target, wherein the ion doping systemcontrols the arc discharge such that a constant amount of arc currentflows between the filament and the anode electrode.
 2. The ion dopingsystem of claim 1, further comprising: a filament power supply forapplying a voltage to the filament; and an arc power supply for applyinga voltage between the filament and the anode electrode, wherein the iondoping system controls the filament power supply and/or the arc powersupply such that a constant amount of arc current flows between thefilament and the anode electrode.
 3. The ion doping system of claim 2,further comprising an ammeter for measuring the arc current, wherein theion doping system controls the output voltage of the arc power supplysuch that the arc current measured with the ammeter has a constantvalue.
 4. The ion doping system of claim 1, wherein a plurality of ionsspecies are produced at a constant ratio from the gas as a result of thearc discharge.
 5. A semiconductor device comprising: a substrate with aninsulating surface; and a crystalline silicon film provided on thesubstrate, wherein the semiconductor device includes a plurality ofsemiconductor components, in each of which source, drain and channelregions are defined in the crystalline silicon film by introducing thedopant element as an impurity into the crystalline silicon film usingthe ion doping system of claim
 1. 6. The semiconductor device of claim5, wherein the semiconductor device satisfies the inequality 0.05≧3σ/Ave, where Ave is the average of dopant concentrations in therespective channel regions of the semiconductor components and σ is thestandard deviation thereof.
 7. The semiconductor, device of claim 5,wherein the semiconductor device satisfies the inequality 0.05≧3σ/Ave,where Ave is the average of dopant concentrations in the respectivesource/drain regions of the semiconductor components and σ is thestandard deviation thereof.
 8. The semiconductor device of claim 5,wherein the crystalline silicon film has been crystallized with acatalyst element that promotes the degree of crystallinity of anamorphous silicon film.
 9. The semiconductor device of claim 8, whereinthe amorphous silicon film has a thickness of 25 nm to 80 nm.
 10. Thesemiconductor device of claim 8, wherein the crystalline silicon filmhas the catalyst element at a concentration of 1×10¹⁶ atoms/cm³ or less.11. The semiconductor device of claim 8, wherein the catalyst element isat least one element selected from the group consisting of nickel,cobalt, palladium, platinum, copper, silver, gold, indium, tin, aluminumand antimony.
 12. The semiconductor device of claim 8, wherein thecatalyst element is nickel.
 13. The semiconductor device of claim 8,wherein the crystalline silicon film has been formed by carrying out atleast one of a furnace heating process, a lamp annealing process and alaser radiation process after the catalyst element has been introduced.14. An ion doping method comprising the steps of: decomposing a gas,including a dopant element, through an arc discharge; and acceleratingions, which have been produced as a result of the step of decomposing,with a predetermined voltage, thereby bombarding a target with the ions,wherein the step of decomposing includes controlling the arc dischargesuch that a constant amount of current is produced by the arc discharge.15. The ion doping method of claim 14, wherein the dopant element isboron or phosphorus.
 16. A method for fabricating a semiconductordevice, the method comprising the steps of: (A) forming an amorphoussilicon film on a substrate with an insulating surface; (B) adding acatalyst element to the amorphous silicon film; (C) thermally treatingand crystallizing the amorphous silicon film, to which the catalystelement has been added, thereby turning the amorphous silicon film intoa crystalline silicon film; (D) generating an arc discharge thatproduces a constant amount of arc current, thereby decomposing a gasincluding an impurity element and accelerating, and introducing into thecrystalline silicon film, ions that have been generated as a result ofthe decomposition; and (E) thermally treating the crystalline siliconfilm.
 17. The method of claim 16, further comprising the step of formingan insulating film on the crystalline silicon film after the step (C)has been performed, wherein the step (D) includes the steps of: (D1)generating the arc discharge that produces the constant amount of arccurrent, thereby decomposing the gas including the impurity element andaccelerating, and introducing into the crystalline silicon film by wayof the insulating film, the ions that have been generated as a result ofthe decomposition; (D2) making a pattern of a conductive material on thecrystalline silicon film; and (D3) generating the arc discharge thatproduces the constant amount of arc current, thereby decomposing the gasincluding the impurity element and accelerating, and introducing intothe crystalline silicon film using the pattern as a mask, the ions thathave been generated as a result of the decomposition.
 18. The method ofclaim 17, wherein the impurity element in the step (D1) is boron and theimpurity element in the step (D2) is phosphorus.
 19. The method of claim16, wherein the catalyst element is at least one element selected fromthe group consisting of nickel, cobalt, palladium, platinum, copper,silver, gold, indium, tin, aluminum and antimony.